Defibrillation is a medical procedure that uses an electrical current to treat life-threatening heart rhythm disturbances, particularly those that cause sudden cardiac arrest. This rapid intervention stops the chaotic electrical activity within the heart, allowing the organ to reset and resume a normal, effective pumping rhythm. The success of this therapy relies on immediately delivering a controlled electrical shock across the chest to the heart muscle.
The Electrical Chaos Requiring Intervention
The need for defibrillation arises from the heart’s electrical system losing its organized control, resulting in two specific lethal rhythms. The most common is Ventricular Fibrillation (VF), a state of severe electrical disorganization in the heart’s lower chambers, the ventricles. Instead of contracting in a coordinated squeeze to pump blood, the ventricular muscle fibers merely quiver, or fibrillate, without producing meaningful blood flow. This chaotic activity immediately leads to a loss of pulse and consciousness.
The second rhythm requiring intervention is pulseless Ventricular Tachycardia (pVT), where the ventricles beat extremely fast but fail to circulate blood. Both VF and pVT require an immediate electrical shock to terminate the chaos and restore systemic circulation.
The Science of Electrical Reset
The mechanism of defibrillation works by momentarily stopping all electrical activity in the heart to allow its natural pacemaker to take over. The electrical shock delivered by the defibrillator simultaneously depolarizes every heart muscle cell. By depolarizing all the cells at once, the shock extinguishes the multiple, disorganized electrical wavelets that perpetuate the fibrillation. This widespread electrical silence allows the Sinoatrial (SA) node, the heart’s natural and highest-ranking pacemaker, to potentially re-establish control.
If successful, the SA node initiates a single, synchronized electrical impulse that travels through the heart in the correct sequence. The goal is to replace the chaotic quivering with a coordinated contraction, restoring a normal heart rhythm that effectively pumps blood. The use of a biphasic waveform, which reverses polarity partway through the shock, lowers the required energy threshold for successful defibrillation compared to older monophasic devices.
Devices Used to Deliver Defibrillation
Defibrillation is delivered using several distinct types of devices, each tailored for a specific environment and user. The first type is the Manual Defibrillator, typically found in hospitals and ambulances and operated by trained medical professionals. These devices require the operator to analyze the heart rhythm on a monitor and manually select the energy level before delivering the shock. Because the user must interpret the rhythm, this device offers the most control to the highly skilled clinician.
The second type, and the most common in public spaces, is the Automated External Defibrillator (AED). These devices are designed for use by lay responders who have minimal or no medical training. The AED uses sophisticated algorithms to automatically analyze the patient’s heart rhythm once the pads are attached and will only advise or deliver a shock if a shockable rhythm like VF is detected. This automation removes the need for rhythm interpretation, making it a powerful tool for rapid intervention in community settings.
The third category includes the Implantable Cardioverter-Defibrillator (ICD), which is a small device surgically placed beneath the skin, similar to a pacemaker. The ICD continuously monitors the heart rhythm and is intended for individuals at high risk for sudden cardiac death. If the internal device detects a life-threatening rhythm like VF, it automatically delivers a corrective electrical shock internally without external intervention.
Differentiating Defibrillation and Cardioversion
While both defibrillation and cardioversion use an electrical shock to terminate an abnormal heart rhythm, the two procedures differ fundamentally in their timing and purpose. Defibrillation is an unsynchronized electrical shock used for emergency, chaotic rhythms, such as ventricular fibrillation, where the patient has no pulse. The shock is delivered immediately because the heart’s electrical activity is too disorganized to time a delivery.
In contrast, cardioversion is a synchronized electrical shock used to treat organized but rapid rhythms, such as atrial fibrillation or atrial flutter, in patients who still have a pulse. The cardioversion device must synchronize the electrical delivery with the peak of the heart’s R wave, which represents ventricular depolarization. This synchronization is performed to avoid delivering the current during the vulnerable T-wave period, which could inadvertently trigger lethal ventricular fibrillation. Cardioversion generally uses lower energy levels than defibrillation and is often a planned procedure performed under sedation.